During the last year, we have generated 60 ES cell lines, in each of which one of a total of 60 different TFs can be overexpressed in tetracycline-inducible manner. We have characterized these ES cell lines as follows: (i) subcellular localization of Flag-tagged transcription factors by immunohistochemistry; (ii) induction levels of the manipulated transcription factors by quantitative RT-PCR, (iii) DNA microarray-based expression profiling before and after induction of the transcription factors; (iv) western blotting, and (v) karyotyping. Together, these results indicate that we have generated truly reliable TF-manipulable ES cell lines. We are currently expanding the collection of ES cell lines.